Postretirement Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (income)	$ (368)	$ 60	$ (845)	$ 119
United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1,149	1,409	3,503	4,251
Expected return on plan assets	(2,590)	(2,435)	(7,761)	(7,309)
Amortization of net actuarial loss	1,045	1,092	3,333	3,196
Net periodic benefit cost (income)	(396)	66	(925)	138
United Kingdom [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	90	111	269	327
Expected return on plan assets	(80)	(137)	(242)	(404)
Amortization of net actuarial loss	18	20	53	58
Net periodic benefit cost (income)	$ 28	$ (6)	$ 80	$ (19)